A magnetic tunnel junction (MTJ) element as a magnetoresistive element has a basic structure that is a stack structure that includes a storage layer having a changeable magnetization direction, a reference layer having a fixed magnetization direction, and an insulating layer provided between the storage layer and the reference layer. Such an MTJ element is known to have a tunneling magnetoresistive (TMR) effect, and is used as the memory element of a memory cell in a magnetoresistive random access memory (MRAM).
An MRAM is a nonvolatile magnetic memory that stores information (“1” or “0”) depending on changes in the relative angle between the magnetization directions of the magnetic layers in each MTJ element. As the magnetization switching speed is several nanoseconds, high-speed data writing and high-speed data reading can be performed. In view of this, MRAMs are expected to be next-generation high-speed nonvolatile memories. Further, where a technique called spin-injection magnetization switching is used to control magnetization with a spin-polarized current, the cell size in an MRAM is reduced, and the current density increases accordingly. With this, the magnetization of each storage layer can be readily switched, and a high-density MRAM that consumes less power can be formed.
To increase the density of a nonvolatile memory, it is preferable to increase the degree of magnetoresistive element integration. Further, to reduce the write current while increasing the degree of integration, MRAMs using MTJ elements containing a magnetic material having a magnetization direction perpendicular to the film plane, or a magnetic material having perpendicular magnetic anisotropy, have been developed in recent years.
Normally, a writing current for switching a magnetization direction by the spin transfer torque switching technique depends on the saturation magnetization and the magnetic relaxation constant of the storage layer. Therefore, there is a demand for a storage layer that has a lower saturation magnetization and a lower magnetic relaxation constant so that the magnetization direction of the storage layer can be switched by low-current spin injection.
Further, as devices become smaller, the devices are more easily affected by thermal disturbance. Therefore, each storage layer is expected to have a high perpendicular magnetic anisotropy. An interfacial layer that is interposed between a storage layer and an insulating layer and is exchange-coupled to the storage layer does not have a sufficiently high perpendicular magnetic anisotropy. Therefore, each interfacial layer is expected to have a high magnetoresistance ratio and a high perpendicular magnetic anisotropy by forming a stack structure with a storage layer.
A storage layer and an interfacial layer each have a magnetoresistance ratio and a perpendicular magnetic anisotropy at a high degree of crystallization, but have different crystalline structures. Therefore, when a storage layer and an interfacial layer are stacked directly on each other, the crystal orientations are disturbed.